Conventionally, a regulator is switched ON by a vehicle key (a "car key"). With a so-called "single function" regulator, the regulator is connected to a contact of the car key via a fault warning lamp.
In more sophisticated regulators, the regulator circuitry is permanently under power, but it must switch from a rest or standby state where it consumes little electricity to an active state when it detects that the key contact has closed.
However, to make the wiring of such regulators compatible with single function regulators, it is not possible to provide a direct link between the key contacts and the regulator, which means that contact closure must be detected via the fault warning lamp.
In practice, the central problem of such detection lies in the discrimination that must be performed between the case where the key contact is open and the lamp is off, in which case the lamp terminal connected to the regulator is grounded, and the case where the key contact is closed but the lamp is on because there is a fault, in which case the terminal of the lamp connected to the regulator is at a potential fixed by the voltage drop of the transistor powering said lamp. The smaller this voltage drop the closer the potential of the lamp terminal connected to the regulator is to ground, thereby making it difficult to discriminate between the open and closed states of the key contact.
One known solution is to increase said voltage drop so as to facilitate discrimination by comparison with a suitable voltage threshold, as described below with reference to FIG. 1.
One terminal of a lamp L is connected via the key switch CL to a voltage B+. The other terminal of the lamp is connected firstly to one or more loads, secondly to the negative input of a differential amplifier A, and thirdly to an input of a comparator CP. The positive input of A receives a threshold voltage VS1, e.g. of 0.9 volts. The second input of CP receives a threshold voltage VS2, which is less than VS1, and is 0.7 volts, for example.
The output of A is looped back to its negative input, while the output of CP constitutes the key detection signal.
When the key switch is closed, amplifier A makes it possible to power the lamp to indicate a fault.
The purpose of the amplifier A is to regulate the voltage drop of the transistor to a value of 0.9 volts. This guarantees that when the lamp is on, the voltage at the terminal SL is always close to said value.
Under such conditions, the comparator CP can discriminate reliably between the situation where SL is grounded (key open circuit) and the situation where the key is closed circuit and the lamp is on, in which case SL is at 0.9 volts, which is greater than the 0.7 volt threshold.
However, with such a known circuit, another problem appears when the voltage at the lamp terminal connected to the regulator is taken as a feed voltage for auxiliary loads CA as shown in FIG. 1, which auxiliary loads may include relays, for example. Thus, an increase in the voltage drop runs the risk of causing such relays to stick ON when the voltage drops from a level close to battery voltage B+ (key switch closed and lamp OFF) to said voltage drop value (key switch closed and lamp ON).
This prior art circuit has two other major drawbacks. Firstly by requiring an amplifier and a comparator it is complex and expensive. Secondly, such a circuit is sensitive to the electrical disturbances that act on the regulation performed by the amplifier, and which are detected by the comparator.
The present invention seeks to mitigate these drawbacks of the prior art.